In the traveling-wire electroerosion system, a continuous wire electrode is axially transported by a wire axial drive means from a supply means to a takeup means. In the path of wire travel between the supply and takeup means, a pair of guide members are disposed at opposite sides of the workpiece to provide a straight line path along which the wire electrode is to travel axially to traverse the workpiece in spaced juxtaposition and precise machining relationship therewith across the machining gap. Tension means is commonly provided to span the wire electrode tightly across the guide members and thereby assures that it travels taut precisely along the said straight line path.
Means is also provided to keep the machining gap flooded with a machining fluid which is essentially of dielectric nature or low conductivity. When an output voltage from an EDM power supply is developed across the gap between the traveling-wire electrode and the workpiece, the fluid filling the gap is broken down to cause an impulsive electrical discharge therethrough, thereby electroerosively removing stock from the workpiece and also material from the wire electrode. The erosive wear of the wire electrode is compensated for by virtue of the continuous travel thereof through the machining gap and the consequent continuous renewal of the machining surface. The EDM power supply is adapted to apply a succession a machining voltage pulses across the machining gap to intermittently produce such electrical discharge, thereby continuing the stock removal from the workpiece. As the stock removal continues, the workpiece is displaced relative to the traveling wire electrode transversely to the longitudinal axis thereof or the aforesaid straight line path. The relative displacement is effected along a prescribed contouring path, typically under commands of a numerical controller (NC) so that a desired contour of cut is generated in the workpiece.
A typical example of the machining fluid which constitutes the discharge medium in the traveling-wire machining system is a deionized aqueous fluid, e.g. water from the municipal supply or tap deionized through an ion-exchange cartridge so that it has an electrical resistivity generally on the order of 10.sup.3 to 10.sup.5 ohm-cm and becomes dielectric in nature. The water fluid has been advantageously employed because of its ready availability and nonflamability in the traveling-wire EDM system. The water fluid has also good cooling ability to dissipate heat developed in the gap regions due to the machining electrical discharges.
In order to assure high cutting accuracy, the traveling wire electrode must be very thin or slender. Customarily, a copper or brass wire of a thickness or diameter as small as 0.05 to 0.5 mm has been employed. The "wire" is customarily circular in cross section but may be of any other cross sectional shape, and may be of a narrow band, taper or ribbon. Copper or brass wire materials have been found to be generally satisfactory because of their relatively high electrical and thermal conductivities in addition to reasonable toughness.
One of major problems in the traveling-wire EDM art is the problem of breakage of the wire electrode which is as thin as mentioned above. It has been recognized that when the electrode wire is excessively heated or insufficiently cooled, the wire tends to break. This is especially true when higher amperage or current density is employed in an attempt to achieve a higher removal rate and cutting speed. Prior efforts in the art to avoid wire breakage have been concentrated on dissipating the discharge heat as quickly and efficiently as possible, say, by using an increased rate of travel of the wire electrode or increase rate of supply of the machining fluid into the gap, but with only limited success. As a consequence, there has been an unsatisfactory limit of the machining current which can be delivered to the machining gap to increase the cutting speed and enhance the machining efficiency. When one attempts to exceed such limit, the wire tends to break.
I have now observed that the phenomenon of wire breakage in the conventional traveling-wire EDM process is, apart from the apparent discharge heat or apparent thermal parameters of electrical discharges, largely related to the generation of gases as a result of decomposition of the aqueous machining fluid in the EDM gap. It has been observed that the aqueous fluid or water supplied into the gap to serve as the discharge medium is decomposed by the electrical discharges thermally, possibly also electrolytically, to form gases which contain mainly hydrogen and the balance oxygen. These gases are triggered by the electrical discharge to cause explosions which involve the creation of intense pressure and heat, which appear to cause breakage of the wire electrode. The higher the machining current delivered, the greater is the amount of gas generated and hence the greater are the generated pressure and heat so that the traveling wire electrode is more easily broken.